Spectrophotometers may be used to measure the color spectrum on the surface of objects, such as paper, paint chips or fabric swatches. A spectrophotometer for reflectance spectrometry usually includes a light source, such as a light bulb (or lamp) and a power source, optics for transferring the light from the light source to the sample, such as fiber optics, lenses and/or mirrors, and optics for collecting the light, which may also include fiber optics, lenses and/or mirrors. The collected light is then transferred to a device for separating the light into its component wavelengths, such as a diffraction grading or a prism, and then to a detector to measure the intensity of one or more of the different wavelengths of the light.
As a reference, the intensity of light generated by the light source may also be transferred by optics to a detector. The signal generated by the detector from the sample light, and the signal generated by the detector from the reference light, may be transferred through a data processing system, such as a computer, and the result displayed or stored. A large variety of configurations and methods are known, and are described in U.S. Pat. Nos. 5,701,175; 5,400,138; 5,319,437; 4,773,761; 3,885,878; and 3,806,256; all of which are incorporated by reference.
The typical lamp variation compensation method for the Reflectometry industry is to divide the signal of the sample by the signal of the lamp reference. In monochromatic applications, a monochromatic reference detector, such as a silicon photodiode, is adequate. In polychromatic applications, the best reference is a detector that matches the polychromatic response of the sample detector so that color shifts due to lamp temperature variations can be compensated. However, this requires great cost (e.g., two spectrophotometers instead of one). A typical compromise in lower cost systems is to use the reference reading from a monochromatic detector in each of the denominations of the polychromatic sample readings to compensate for lamp variations. It would nevertheless be desirable to have a method which could accurately compensate for variation in lamp temperature which uses only a single reference detector.